The present invention relates to hot melt adhesives, and more particularly, to a method of utilizing microwave activatable hot melt adhesives in the manufacture of disposable nonwoven articles.
Nonwoven fabric is comprised of an interlocking fiber network, and is employed in the construction of disposable goods. Specific applications of nonwovens have included disposable diapers, sanitary napkins, pantyshields, surgical drapes, hospital pads, adult incontinence products and the like.
In such applications it is generally necessary to adhere a substrate composed of nonwoven material, tissue, absorbent fluff, superabsorbent materials, elastic materials or the like to another substrate. This second substrate may be another nonwoven fabric, tissue, fluff, superabsorbent core, or polyolefin materials such as a polyethylene or polypropylene layer. Typically, a hot melt adhesive has been used to bond such substrates together since there is no evaporation step necessary during manufacture, as would be the case for water-based or solvent-based adhesives. Suitable hot melt adhesives must posses the appropriate bond strength to adhere the substrates involved, and must also possess good flexibility, no staining or bleedthrough, suitable viscosity and open time to function on commercial equipment, acceptable stability under storage conditions, and acceptable thermal stability under normal application conditions.
Many different polymers have been used in hot melt adhesives employed in the construction of disposable nonwoven goods. In this regard, typical hot melt adhesives have employed polymers which have included S-I-S (styrene-isoprene-styrene); SBS (styrene-butadiene-styrene); SEBS (styrene-ethylene-butylene-styrene); EVA (ethylene-vinyl-acetate); and APAO (amorphous-poly-alpha-olefin). While these polymers, when properly blended, provide acceptable adhesion between most substrates employed in typical nonwoven construction such as diapers, they have had several shortcomings which have detracted from their usefulness.
One of the most noteworthy shortcomings of prior hot melt adhesives concerns open time, i.e. the time between application of the hot melt adhesive onto a substrate and before contact with the second substrate to which the first substrate is to be bonded. This time period results in cooling of the adhesive, and thus less flow of the adhesive occurs when in contact with the other substrate. This results in a weaker bond than desired in many circumstances. Thus, it is important in nonwoven constructions to insure that the hot melt adhesive is as hot as possible when in contact with the two substrates to be bonded so that sufficient flow of the adhesive into the substrates occurs resulting in the strongest possible bonds.
Another shortcoming of prior art hot melt adhesives concerns thermal stability under normal application conditions. Since hot melt adhesives must be melted prior to application, many hot melt adhesives cannot be used for bonding substrates which are typically employed in the construction of nonwoven articles due to their lack of heat stability. Thus, although many hot melt adhesives are known that would provide strong bonds with substrates such as polyethylene, polypropylene, nonwoven materials, tissue, fluff or the like, they are not typically employed in the construction of nonwoven articles due to unacceptable thermal stability. Thus, it would be desirable to utilize a method in the construction of nonwoven articles which would allow for the use of hot melt adhesives without regard to their heat stability.
Microwave activatable hot melt adhesives are known in the art. Attempts to render hot melt adhesives more sensitive to microwave radiation involve the addition of inorganic materials such as carbon fibers, carbon black or graphite to aid base hot melt adhesive such as that proposed in U.S. Pat. No. 4,906,497. Another method is disclosed in U.S. Pat. No. 5,238,975 which incorporates metals such as tungsten, chromium, aluminum, copper, titanium, titanium nitride, molybdenum disilicide, iron and nickel. Either of these approaches, however, has serious limitations. Some of these problems include the fact that the added materials settle out in the melt due to their weight, and may cause pump abrasion as well as filter or nozzle clogging. Some also represent potential fire and explosion hazards.